fix(redesign): contain modal scroll chaining and smooth Telegram chat refresh

- Add overscroll-behavior: contain to all in-modal/popup scroll
  containers (Modal body, EntitySelect, MultiEntitySelect, IconPicker,
  IconGridSelect, SearchPalette, TimezoneSelector) so reaching the
  inner scroll boundary no longer scrolls the page underneath.
- Telegram bot Discover Chats no longer collapses the existing chat
  list into a "Loading…" placeholder. Split chatsLoading (initial)
  from chatsRefreshing (Discover); rows are keyed by chat.id with
  flip+fade animations; the list dims with a sweeping shimmer bar
  while the Discover button shows a spinning icon and "Discovering
  chats…" label. Honors prefers-reduced-motion.

frontend/src/lib/components/EntitySelect.svelte

@@ -304,6 +304,7 @@
 	/* List */
 	.ep-list {
 		overflow-y: auto;
+		overscroll-behavior: contain;
 		scrollbar-width: thin;
 		padding: 0.35rem;
 		position: relative;